The present invention relates to a motor-driven injection driving apparatus for an injection molding machine, in which a driving portion is made small in size by means of synchronous driving using a plurality of motors to enhance the arrangement balance of driving force and functional parts.
A conventional motor-driven injection driving apparatus for an injection molding machine is provided with one or two drive motors. For the injection driving apparatus provided with one drive motor, two pairs of screws and nuts are used to convert the rotational motion of the motor into linear motion, and these pairs are arranged at symmetrical positions on both sides of an injection screw. The power is transmitted by a toothed transmission belt or a gear train (refer to an injection apparatus for an injection molding machine disclosed in Japanese Patent Publication No. 8-9184 (No. 9184/1996), for example).
On the other hand, Japanese Patent Publication No. 3-41050 (No.41050/1991) has disclosed an injections crew driving apparatus in which the driving force is distributed so that a standard motor can be used. In this driving apparatus, individual driving servomotors are connected directly to screw shafts for linear driving provided on both sides of an injection screw.
In the aforementioned motor-driven injection driving apparatus, in order to avoid an overload of motor, protecting means must be taken: for example, a resin pressure is measured at the time of injection, and if the resin pressure exceeds the standard value, the injection speed is reduced. For the protecting means disclosed in Japanese Patent Provisional Publication No. 62-218118 (No. 218118/1987), a strain gage is provided at the connecting portion between driving means for driving a screw for an injection molding machine in the axial direction and the screw, and a pressure applied to a resin in a mold is detected by this strain gage.
The injection molding machine of a medium or larger size requires a very high pressure for injection. Therefore, when a conventional injection driving apparatus using one motor is applied to the injection molding machine of a medium or larger size, a motor of special specifications is needed to deliver a high torque, so that the cost increases, and also the arrangement balance becomes bad when the apparatus is mounted on the injection molding machine.
On the other hand, for the injection driving apparatus using two injection drive motors, the rotation of motor is kept low, so that the output of motor is also kept low. Therefore, in order to provide a necessary injection pressure, a large motor is used, which leads to the increase in cost and weight.
Also, in the arrangement of screw shaft in the injection driving apparatus disclosed in Japanese Patent Publication No. 3-41050 (No.41050/1991), a compressive force acts on the screwshaft from the nut side in the injection process. Therefore, to stably support the screw shaft, a bearing is provided to support the end portion of the screw shaft. Nevertheless, the screw shaft is pushed and bent, so that buckling may occur. Also, there arises a problem in that if a bearing is provided at the end portion of the screw shaft, the overall length of the injection apparatus increases.
The present invention has been made to solve the above problems with the prior arts, and accordingly an object thereof is to provide a motor-driven injection driving apparatus for an injection molding machine, in which a small standard motor is used as an injection drive motor to decrease the cost, and the apparatus can be mounted on the injection molding motor with a good balance.
Also, another object of the present invention is to provide a motor-driven injection driving apparatus for an injection molding machine, in which inexpensive pressure detecting means of an ordinary size can be used, the performance of the pressure detecting means can be maintained for a long period of time, and the injection pressure in the injection and dwell process can easily be controlled with a closed loop.
The present invention provides a motor-driven injection driving apparatus for an injection molding machine, which is configured so that the rotation of an electric motor is converted into linear motion, and an injection screw is moved forward and rearward by the linear motion, comprising: a fixed frame provided with an injection cylinder in which the injection screw is fitted so as to be moved forward and rearward; first and second injection drive motors installed on the fixed frame; first and second ball screw shafts pivotally supported on both sides of the fixed frame so as to be in parallel and symmetrical with respect to the axis of the injection cylinder and so as to be rotatable; first and second power transmission mechanisms for transmitting the rotational forces of the first and second injection driving motors to the first and second ball screw shafts, respectively, while reducing the speed; a movable frame which is provided, on both sides, with first and second ball screw nuts threaded on ball threads of the first and second ball screw shafts, respectively, and is capable of being moved in the direction of injection together with the injection screw; a screw drive motor, which is mounted on the movable frame, for rotatively driving the injection screw; and a controller for synchronously rotating the first and second injection drive motors, in which the movable frame is moved so that a tensile force acts on the first and second ball screw shafts in an injection process.
Therefore, since the rotational speed of the injection drive motor is increased by means of the speed reduction operation of the power transmission mechanism, whereby the output efficiency of the motor can be enhanced, a small standard motor can be used as the injection drive motor to decrease the cost. Also, the arrangement balance is improved when the apparatus is mounted on the injection molding machine.
Further, since a large force necessary in the injection process acts on the first and second ball screw shafts as a tensile force, there is no fear of deflecting the first and second ball screw shafts. Moreover, since the actuating force in the return direction of the movable frame is far smaller than the actuating force at the time of injection, the ball screw shafts can be moved stably without supporting one end of each of the ball screw shafts by a bearing. As a result, the cost can be decreased and the construction can be simplified.
Also, in the present invention, first and second intermediate bearing frames are provided on one side and on other side of the fixed frame, and the power transmission mechanism includes an intermediate transmission shaft pivotally supported on the Intermediate bearing frame so as to be rotatable and winding transmission means for transmitting power from the injection drive motor to the ball screw shaft through the intermediate transmission shaft while reducing the speed.
Therefore, the rotation of the injection drive motor is transmitted to the ball screw shafts by the power transmission mechanism while the speed is reduced at two stages. As a result, the rotational speed of the motor can be increased, whereby the output efficiency of the motor can further be enhanced.
Further, in the present invention, the power transmission mechanism includes a planetary reduction gear connected directly to the injection drive motor, a small pulley for transmission belt, which is fixed to an output shaft of the planetary reduction gear, a large pulley for transmission belt, which is fixed to the ball screw shaft, and a transmission belt set around the small and large pulleys. Therefore, the whole shape of the power transmission mechanism can be made compact.
Also, the present invention provides a motor-driven injection driving apparatus for an injection molding machine, which is configured so that the rotation of an electric motor is converted into linear motion, and an injection screw is moved forward and rearward by the linear motion, comprising: a fixed frame provided with an injection cylinder in which the injection screw is fitted so as to be moved forward and rearward; first and second injection drive motors installed on the fixed frame; first and second ball screw shafts which are pivotally supported on both sides of the fixed frame so as to be in parallel and symmetrical with respect to the axis of the injection cylinder and so as to be rotatable, and are driven by the first and second injection drive motors, respectively; a movable frame capable of being moved in the direction of injection together with the injection screw; a screw drive motor, which is mounted on the movable frame, for rotatively driving the injection screw; first and second ball screw nuts threaded on ball threads of the first and second ball screw shafts, respectively; a pressure detecting sensor interposed between the movable frame and each of the ball screw shafts; and control means for providing feedback control of a screw back pressure, injection speed, and dwell pressure after injection so that the pressure detected by the pressure detecting sensor becomes a preset injection pressure.
Therefore, the first and second ball screw shafts carry a large injection force that has been distributed, and the distributed injection forces are detected by the pressure detecting sensors, so that a low-capacity pressure detecting sensor can be used.
Also, when the first and second ball screw shafts are turned fully synchronously, and the load is equally distributed to these ball screw shafts, the feedback control of pressure can be carried out by detecting only the load acting on one of the ball screw shafts. That is to say, the feedback control of pressure using only one pressure detecting sensor can be provided. If the feedback control using a single pressure detecting sensor is carried out, the cost can be reduced.
If a sensor of load cell type, in which a small displacement of a highly rigid cell body is measured by a strain gage, is used as the pressure detecting sensor, sufficient durability of the pressure detecting sensor can be maintained.